We Have Made Contact!

I think most of you know my real job is working with FEA and Solid Edge Simulation. One of the questions I sometimes get is about modeling contact between assemblies.For Linear static problems, Solid Edge Simulation supports both NX Nastran's Glue and Linear contact. I put together a simple example to show the difference.

Below are two iron parts both constrained at their back sides (shown by blue spheres). In this model, I'm going to put a 1000lb force on the top part to see how the model reacts. As you might guess, the top part will bend and push into the bottom part. How should we model this?

If we do nothing, the top part will simply pass through the bottom part as the elements at the top part don't know anything about the elements at the top. Obviously, this is not the right answer.

The easiest solution is to use glued contact. This tells NX Nastran that any of the elements that are touching each other should transfer the forces to the elements on the other side. The problem with a glued connection, as you might guess, is that it doesn't take into account when the nodes and elements might pull away from each other either by separating or sliding. If your parts really are glued or welded, this is good. If not, you will get the wrong answer.

The next option is to use NX Nastran's Linear Contact. This allows nodes and elements to separate or slide. This is a more complicated problem for NX Nastran to solve and will take longer than using the glued connection. It can also be more confusing to the new user. A glue contact will fully constrain the glued part while the linear contact will not. So a model that solved with glued contact may start telling you your model is not fully constrained when you switch to linear contact.

Note that the maximum stress location in both the glued and linear contact is at roughly the same location but the stress differs considerably for exactly the same load, constraints, and parts.

Solid Edge Simulation can output video files so I took a few examples and included them on a youtube. It includes the options above as well as some other ways to display results.